Conduction System Pacing in Pediatrics and Congenital Heart Disease: A Case Report and Literature Review

Conduction system pacing involving either His bundle pacing (HBP) or left bundle branch pacing (LBBP) is a modality that has been introduced as a safe and effective alternative to right ventricular (RV) pacing to help prevent pacemaker-associated cardiomyopathy. While HBP has been employed in the pediatric and congenital populations, several small studies have shown LBBP to be safe and effective in the pediatric population. We present a patient with congenital atrioventricular block and postoperative ventricular septal defect repair cardiomyopathy with subsequent left ventricular function improvement following a transition from an RV epicardial pacemaker system to an LBBP system. This case report serves as a foundation for a review of the current state of LBBP in pediatrics and congenital heart disease.


Introduction
His bundle pacing (HBP) and left bundle branch pacing (LBBP), collectively considered under the broader classification of conduction system pacing (CSP), have been introduced as alternatives to biventricular (BiV) cardiac resynchronization therapy (CRT).CRT has long been shown to improve BiV function in adult patients with heart failure 1 ; however, this procedure is often not a practicable pacing strategy in the pediatric population. 2 Although both HBP and LBBP have been shown to be useful in the potential prevention of pacemaker-associated cardiomyopathy, LBBP implantation may constitute a technically more successful and reliable strategy. 3,4Informed consent was obtained from the patient's parents via telephone by the authors.The Indiana University/Indiana University Health Institutional Review Board deemed this case report and literature review exempt from review.

Case presentation
A 15-year-old boy with a history including congenital atrioventricular (AV) block with secundum atrial and perimembranous ventricular septal defects (ASD, VSD) underwent single ventricular lead epicardial pacemaker placement at 2 weeks of life.His device was subsequently replaced with a dual-chamber epicardial system with DDD programming at 6 months during VSD and ASD repair.At 9 years of age, he presented with a ventricular lead fracture, underwent epicardial ventricular lead revision, and continued with DDD pacing (Figure 1).His echocardiogram at that time showed mildly decreased left ventricular (LV) systolic function with an ejection fraction (EF) of 52% and a moderately dilated left ventricle.He was treated with goal-directed medical therapy, including an angiotensin-converting enzyme inhibitor/ angiotensin II receptor blocker and β-blocker, as the function continued to decrease in the coming years.At 15 years of age, he had moderately decreased LV systolic function with an EF of 37% in the setting of chronic RV pacing and poor medication adherence.Despite these echocardiographic findings, he continued to be asymptomatic and played competitive soccer for his high school and club teams.
The patient subsequently presented to the emergency department with bradycardia.His device interrogation demonstrated no atrial tracking due to no sensing and an increased atrial lead impedance that was consistent with an atrial lead fracture.His device was reprogrammed from DDD 45-210 ppm to ventricular demand pacing, VVIR 60-175 ppm.At follow-up evaluation, he complained of increasingly frequent positional dizziness.He was inconsistent in his use of home medications, which included of carvedilol and losartan.His heart rate was 63 bpm, and his blood pressure was 118/58 mmHg.His physical examination revealed no evidence of congestive heart failure.Electrocardiography showed an asynchronous ventricular-paced rhythm with a QRS duration of 188 ms and a left bundle branch block (LBBB) pattern ( Figure 2).Echocardiography revealed mild dilation of the left ventricle with a moderately decreased EF of 37%, as well as previously noted paradoxical septal motion and no residual ASD and VSD.A device interrogation showed normal ventricular lead function with no evidence of tachyarrhythmia.Due to the setting of new-onset symptoms, progressive cardiomyopathy, and poor medication compliance, the decision was made to revise his pacing system to a transvenous system with the advantages of AV synchrony, endocardial ventricular activation, and ideally a conduction system pacing lead.
In the operating room, the patient was positioned supine and placed under general anesthesia.A transverse incision was made beneath the left clavicle, and electrocautery dissection was performed to expose the fascia above the pectoralis major.A pocket was created above the muscle with a blunt dissection.The left axillary vein was accessed using the modified Seldinger technique in two adjacent locations, and guidewires were advanced to the right atrium.A short tear-away sheath was then placed over the lateral guidewire.A SelectSite C315HIS (Medtronic Inc., Minneapolis, MN, USA) catheter was then advanced over the guidewire into the right atrium, and the guidewire was removed thereafter.A 3830-59 lead (Medtronic Inc.) lead was advanced through the catheter into the RV.There was no clear His potential when pulling back toward the tricuspid valve annulus.The catheter and lead were advanced, and the lead was then secured into the ventricular septum at the location of the left bundle.Additional lead rotations were completed under fluoroscopic guidance while maintaining catheter contact with the septum to demonstrate intraseptal lead tip location.Testing confirmed appropriate positioning of the lead with a stimulation to QRS of 41 ms and a QRS duration of 86 ms as measured by the Medtronic analyzer and the electrophysiology laboratory recording system.The SelectSite catheter was removed using the preferred spring-loaded slitting device while observing the lead tip using fluoroscopy.The short sheath was then removed without incident, and the lead was secured with a non-absorbable suture.A SelectSite C315-J (Medtronic Inc.) catheter was advanced through a separate short sheath to the right atrium, and a 3830-59 (Medtronic Inc.) lead was then advanced and secured in the right atrial appendage.Testing confirmed the position and adequate sensing and pacing thresholds.The catheter and sheath were removed, and the lead was secured.The previous abdominal generator was then removed without difficulty.The transvenous leads were subsequently connected to the generator, which was placed in the left infra clavicular pocket.A new generator (DDD 40-210) was programmed once the new leads were tested (Figure 3).Postoperative surface electrocardiography showed an atrial-sensed ventricular-paced rhythm with a QRS duration of 98 ms (Figure 4A).The patient tolerated the surgery well and was discharged the next day.At the first ambulatory follow-up evaluation, the patient reported improved energy and no orthostatic symptoms.Electrocardiography again showed an atrial-sensed, ventricular-paced rhythm with a QRS duration of 114 ms, right bundle branch block (RBBB) paced QRS morphology, and resolution of T-wave memory (Figure 4B).The atrial lead impedance was 532 Ω, and the ventricular lead impedance was 570 Ω.The P-wave amplitude was 5 mV, and the R-wave amplitude was >20 mV.The atrial threshold was 0.5 V at 0.4 ms, and the ventricular threshold was 0.5 V at 0.4 ms.An echocardiogram recorded just under 4 weeks postoperatively showed stable mild LV dilation with an improved EF of 48%.

Discussion and literature review
The discussion surrounding CSP as the default strategy among pacemaker implanters continues to evolve.HBP and CSP in the pediatric and congenital heart population have shown some acceptance in the literature. 2,5We reviewed the current literature, focusing on the evolution of LBBP in adult and pediatric patients with and without congenital heart disease (CHD).A literature search was performed with the assistance of a medical librarian using the Medline (Ovid) database with keywords: "selective OR left OR bundle OR pacing" AND "cardiomyopathy outcomes."These same parameters were used to complete an independent literature search in PubMed to identify additional studies missed during the first search.

Cardiac resynchronization therapy and conduction system pacing
][8][9] CRT has been shown to ameliorate cardiomyopathy in the adult population. 1 However, this pacing strategy is not always feasible in the pediatric population due to patients' size and congenital malformations. 2 Dubin et al. studied CRT in the pediatric population, and, although systolic function improved, there were more technical adverse events recorded, including dissection, perforation, or lead dislodgement associated with cardiac venous leads, when compared to adults. 10There was also a wide variability in technical approaches, with half of the devices being epicardial or mixed (epicardial/endocardial) approaches versus transvenous placement. 11,12easons for using an epicardial lead system versus a transvenous approach include tricuspid valve abnormalities, intracardiac shunting, and venous anomalies. 10e CSP has been shown to be non-inferior to BiV pacing (BVP) in adults with a CRT indication. 13However, a recent study by Vijayaraman et al. showed that the primary outcome rate of developing heart failure hospitalization or time to death was significantly lower in the left bundle branch area pacing (LBBAP) group compared to the BVP group. 14Gordon et al. reported the largest case series of CSP in patients of pediatric age in 2022. 4They observed improvements in pacemaker-associated cardiomyopathy following implantation of LBBP devices.This study included 24 patients (12 with CHD and 12 without) who underwent physiologic pacing with a conduction system lead.Four patients had decreased EFs of 32%-45% prior to physiologic pacing and showed intraoperative improvements of 5%-10% that persisted on subsequent serial echocardiograms.Patients were followed for a median of 610 days in this study.The results suggest an improvement in EF with the use of CSP; however, because the study was retrospective, it is indeterminate if CSP can prevent pacing-associated cardiomyopathy.Relative to HBP, selective LBBP systems appear to be simpler to implant, have increased generator longevity due to lower pacing thresholds, and have a greater amplitude of sensed R-waves, making pacing management easier. 3s congenitally corrected transposition of the great arteries (CCTGA) is known to be associated with the AV block, patients with this condition often require permanent pacemakers.][16][17][18][19] These studies have shown both HBP and LBBP to be safe and effective in the setting of CCTGA.

Procedure: left bundle branch pacing
The first and only lead approved for LBBP in bradycardia patients in the United States is the SelectSecure MRI SureScan Model 3830 (Medtronic Inc.). 20Stylet-driven pacing lead systems from Abbott Laboratories (Chicago, IL, USA), Boston Scientific (Marlborough, MA, USA), and Biotronik (Berlin, Germany) have also been used for LBBP. 21There are several reported methods for placing LBBP leads.One approach involves using three-dimensional mapping catheters designed for electrophysiology studies.In this approach, an octopolar or decapolar catheter is generally used to mark the location of the His bundle by identifying the His bundle electrogram and annotating the catheter position using fluoroscopy and/or three-dimensional mapping software.The catheter may be placed via the femoral vein or right axillary vein.Once this location is identified, the lead is positioned just distal to this signal. 4Of note, some operators prefer to place the catheter deeper in the RV and complete at least 10 turns of the 3830 lead instead of the manufacturer-recommended 4-6 turns. 12Other methods include using fluoroscopy and electrocardiography only with or without the use of premature ventricular contraction morphology.The LBB area is located generally 10-20 mm from the His potential location toward the ventricular apex.The lead is positioned in the RV septum along the line between the HBP site and the RV apex in the right anterior oblique (RAO) 30° view.Pacing at high output is performed to evaluate capture prior to rotating the lead deep into the septum. 2reshaping of the C315 catheter to better reach the His bundle may be advantageous for patients of smaller statures. 4Deflectable catheters are also available for this purpose.As noted above, stylet-driven leads have been used to help guide the lead to the left bundle area as well.
Criteria to confirm LBB capture, adapted from the work of Hua et al., 22 include the following: 1. Paced QRS morphology with an RBBB-like pattern 2. Recording of an LBB potential, abrupt shortening of R-wave peak times in V 6 by >10 ms at high output pacing during lead advancement from the mid-septal region to the LV endocardium and then remaining short (usually ≤80 ms) and constant at high and low output 3. Demonstration of capture If criterion 1 in conjunction with either criterion 2 or criterion 3 is met, then the procedure is considered successful.

Application of left bundle branch pacing in adults
The LBBP was first clinically described by Huang et al. in 2017.In this case report, a 72-year-old patient with heart failure and dilated cardiomyopathy experienced an EF improvement from 32% to 62% at 1 year of follow-up. 23ubsequent research has supported long-term improvements in symptoms and EF recovery with LBBP. 24Tan et al. performed a meta-analysis of LBBP in the adult population in which the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines for systematic review were used.The study only included LBBP in patients with a CRT indication.The primary outcomes consisted of QRS duration after CRT device implantation, LV end-diastolic diameter, the New York Heart Association classification, and LVEF.Eight studies with a total of 527 patients meeting the criteria for LBBP for CRT were reviewed.The meta-analysis showed that LBBP provided physiological pacing with narrower QRS durations in patients with LBBB and heart failure with reduced EF when compared to BVP. 25 Ravi et al. performed a single-center study evaluating LBBP performance and lead-related complications during follow-up.A total of 59 patients were reviewed in this retrospective cohort study, and the most common indication for LBBP lead implantation was intraoperative high HBP lead thresholds.Overall, there were seven lead-related complications, and three patients required lead revision, including two with macrolead dislodgements and one with an interventricular septal perforation.The latter lead was successfully explanted under transesophageal echocardiography guidance, and a new LBBP lead was implanted.Overall, 97% of patients had successful implantation of LBBP systems. 26None of the studies that were reviewed included a randomized controlled trial.

Left bundle branch pacing in pediatrics and children with right ventricular pacing-associated cardiomyopathy
8][29] A recent report highlighted a patient with hypertrophic cardiomyopathy and a subcutaneous implantable cardiac defibrillator who developed complete AV block and underwent subsequent successful implantation of an LBBP system with a separate generator. 30Wenlong et al. studied 12 pediatric patients who underwent implantation or replacement of an LBBP pacemaker. 31Eleven of the children had complete AV block, and four cases were secondary to CHD.One patient had cardiac dysfunction secondary to apical RV pacing requiring CRT.The study was completed over a 2-year period.All 12 patients underwent successful implantation of an LBBP device, and the patient with apical RV pacing cardiomyopathy showed improved EF following LBBP.A more recent retrospective study compared children with complete AV block to assess postoperative outcomes of those with RV pacing versus LBBP. 32 total of 48 children were studied and LBBP produced a narrow QRS duration and improved ventricular synchrony as well as a decrease in the time-to-peak longitudinal strain and the average interventricular mechanical delay time postoperatively.LBBP and HBP have been studied together in the pediatric CHD population, but the modalities themselves were not directly compared. 12Jimenez et al. studied eight pediatric patients who underwent CSP with LBBP or HBP and noted a reversal of pacemaker-induced cardiomyopathy in all patients.Patients were not followed beyond initial follow-up, and further long-term studies are needed.

Left bundle branch pacing versus His bundle pacing
With the advent of multiple types of CSP as an alternative to traditional CRT, further long-term studies directly comparing the strategies will be instructive.As previously discussed, limitations of HBP include lead instability/dislodgment and progressive increases in pacing thresholds that may be associated with loss of capture after implantation. 33We believe that the difficult technical aspects of HBP and postoperative complications make LBBP a better option for most pediatric patients, including those with CHD.As technology advances and comfort and familiarity in placing these systems grows, further research comparing the two modalities is warranted.Given the small volume of implanted pacing systems in children and patients with CHD, conducting large trials to answer management issues (as is common in the adult literature) is not generally practicable in the pediatric and congenital heart electrophysiology community.A multicenter registry among pediatric heart centers may offer a viable means to serve this purpose.

Conclusion
While HBP has been employed in the pediatric and congenital populations, several small studies have shown LBBP to be safe and effective in the pediatric population.LBBP may represent a more compelling strategy with improved lead characteristics while maintaining improved LV function offered by CSP in pediatric and CHD patients.A multicenter pediatric and CHD registry may help to explore optimal pacing strategies.

Figure 1 :
Figure 1: The postero-anterior and lateral chest X-ray with epicardial right ventricular pacing system 2 years prior to presentation.Note the right ventricular basal location of the ventricular lead.

Figure 2 :
Figure 2: A 12-lead electocardiogram with an A-sensed, V-paced right ventricular epicardial pacing system demonstrating a left bundle branch block paced QRS morphology with prolonged QRS duration of 188 ms.

Figure 3 :
Figure 3: The postero-anterior and lateral chest X-ray post-transvenous left bundle branch system and atrial epicardial pacing lead fracture.